Table 1.

Explanation of ratings for four key parameters.

Rock mass properties: maximum total rating = 50
Fracture spacing (block size) (maximum rating = 35)Fracture aperture (maximum rating = 15)
Crucially, fracture spacing should include ALL discontinuities deemed to have a tensile strength less than that of the intact rock even if these are incipient hairline cracks (e.g. Selby 1980) or potential weathering lines (Whalley et al. 1982). So, discontinuities induced by excavation, weathering, rebound or other anthropogenic causes should be included along with bedding planes, joints, faults and other lithological fractures. Discontinuity spacing can be determined using orthogonal scanlines or by estimating mean block size from visual assessmentFracture aperture is extremely important in weathering, allowing root growth, water flow, block wedging, wall weathering and dissolution, providing chutes for material redistribution and contributing to the general ‘looseness’ of the rock mass. The shear strength of fracture walls is much less important in deterioration than for slope instability. Once a fracture is open, any subsequent increase in aperture has relatively little impact. Hence, the largest increase in ratings occurs for narrow apertures. Aperture should be measured in relation to separation of fracture walls regardless of infilling (unless the fracture is healed)
Rock material properties: maximum total rating = 50
Rock compressive strength (maximum rating = 35)Rock material weathering grade (maximum rating = 15)
Intact rock strength is considered much less important in rock mass classifications than discontinuity spacing (e.g. Bieniawski 1973; Hack & Price 1993). However, rock strength is of equal importance in deterioration, as weathering processes probably attack rock material as much as the discontinuities contained within it. Rock strength also reflects rock properties such as texture, density and porosity and is a reasonable surrogate for durability. There are a variety of ways to determine or estimate compressive strength in the field and laboratory (e.g. BS 5930: British Standards Institution 1999)The assessment of material weathering should be based on the static weathering condition of intact rock, not discontinuity walls. The classification system adopted is that produced by the Geological Society Engineering Geology Working Party (1995) to describe uniform materials (Approach 2) and is based on Moye (1955). Ratings to be applied are as follows: fresh 0; slightly weathered 5; moderately weathered 10; highly weathered 14; completely weathered 15. Intermediate values can be used as appropriate